(123e) Chlorine-Mediated Electrochemical Ethylene Oxidation in Saline Water

Electrochemically generated chlorine acts as a redox mediator for the selective oxidation of ethylene to 2-chloroethanol, which converts to ethylene oxide in alkaline aqueous electrolyte. This strategy utilizes abundant saline water as an electrolyte and source of oxygen atoms for functionalization. In this system, chlorine mediates the reaction without being consumed, and the generation of the oxidizing agent and its subsequent reactions proceed in single unit operation. We present a mechanistic study of ethylene oxidation in saline water using cobalt oxide nanoparticle catalysts. Electrochemical kinetic analysis suggests that the resting state of the catalyst and the rate-determining step differ for the chlorine evolution reaction in the presence and absence of ethylene. In situ X-ray absorption and Raman spectroscopy provide evidence indicating the ethylene-saturated environment augments the emergence and the abundance of the Co-Cl intermediates. We also discuss practical aspects of chlorine-mediated electrochemical ethylene oxidation. In 0.6 M NaCl pH 8 electrolyte, which resembles seawater, the average current density was ∼60 mA/cm² with a Faradaic efficiency of ∼41% toward ethylene functionalization. Faradaic efficiencies above 70% is achieved when the electrolyte is the seawater containing Mg²⁺ ions. The precipitation of Mg(OH)₂ on the cathode lowers the electrolyte pH, hence the competing oxygen evolution becomes less favorable, which enables high selectivity toward 2-chloroethanol.